1. Field of the Invention
The present invention relates to an apparatus for rearranging data in each field of a digital video signal prior to recording that signal on a recording medium, to reduce the possibility that digital samples which are located closely adjacent in a horizontal scanning line of the video signal will be simultaneously lost as a result of occurrence of a burst error in the recording/playback process.
2. Description of the Related Art
When a digital video signal is recorded and subsequently played back, using a recording medium such as magnetic tape or a magnetic disc, read errors will occasionally occur in one or more tracks of the recording medium, due to such factors as dust or scratches on the surface of the recording medium, etc. When such a read error occurs, one or more digital samples will be lost, i.e. an erroneous value for that sample will appear on the recording medium. The occurrence and positions of such errors are detected at the time of playback by utilizing error detection data that are recorded together with the digital video signal samples. Since there is usually a very high degree of correlation between each sample and samples which are located closely adjacent to it in the resultant playback video picture, it is possible to compensate for such a read error by deriving an interpolated value for the lost sample, using the values of these adjacent samples. If that is done, the effects of such read errors can be effectively prevented from affecting the resultant displayed playback video picture. Such interpolation to minimize the display effects of read errors will be referred to as "retouching", since it is analagous to photographic retouching which is executed to hide small blemishes in a photograph. Usually, the adjacent samples that are used in that processing (with respect to the positions of the samples in a displayed picture) consist of the closest sample in the horizontal scan line immediately above line containing the sample that is lost, the closest sample in the line immediately below that containing the lost sample, the two samples located immediately to the left of the lost sample, in the same line as the lost sample, and the two samples located immediately to the right of the lost sample, in the same line. In practice, a combination of two or more of these adjacent samples, which is found to provide optimum correction for the specific sample that has been lost, is selected to be used in the "retouching" processing. Thus in order to ensure that optimum compensation can be applied for any lost sample, it is essential that all of these adjacent samples be available, for use in the "retouching" processing, so that to achieve optimum compensation for read erros, it is necessary to ensure that all of the aforementioned set of samples which are located closely adjacent to a sample that has been lost due to a read error will be available for use in that "retouching" compensation. In particular, it is necessary to ensure that the adjacent samples that are in the same line of the video signal as a sample that has been lost due to a burst error (i.e. which extends along a recording track over a number of successive samples) will not also be lost as a result of that same burst error.
To attempt to ensure that such a condition will not occur, proposals have been made in the prior art for rearranging the order of the samples of the digital video signal prior to recording, such as to ensure that samples which will be located closely mutually adjacent in the displayed picture will be spaced mutually far apart on the recording medium. Such a method of digital video signal recording, employing rearrangement of sample numbers (i.e. numbers which express the respective positions of the samples in the original digital video signal) is referred to as "shuffling".
If a digital video signal has been recorded after executing such shuffling processing, then ideally at the time of playback, even if some samples have been lost, normal reproduction can be achieved by executing interpolation processing as described above, since samples which are required in that interpolation processing will not have been lost (i.e. due to the fact that these were located far apart from the lost sample, on the recording medium).
In a digital VTR, alternately occurring samples in each line of the video signal are separated and processed in two different recording channels, referred to in the following as channel 0 and channel 1. The channel 0 and channel 1 samples are recorded on mutually different tracks. Hence a burst error on one track, resulting in the loss of a number of samples recorded on that track, cannot result in loss of the pair of samples located immediately adjacent to each lost sample and in the same line, since these most closely adjacent samples will be recorded (through a different channel) on different tracks from that on which the burst error occurred. However there is a high probability that the two samples that are in the same channel and the same line as a sample that has been lost due to a burst error will also be lost as a result of that same burst error. Hence these samples of the same line and same channel will not be available for use in "retouching" interpolation processing to compensate for each sample that has been lost.
A prior art recording technique, which attempts to provide a shuffling method whereby that problem will be overcome, is described in an article entitled "19-mm Type D-2 Composite Format Helical Data and Control Records", in the SMPTE Journal, July, 1990. With a helical scan recording type of digital VTR, due to the fact that the amount of information in each field of the video signal is extremely large, the samples of each field are divided among a plurality of tracks on the recording tape. Each sample is assigned a channel number and a segment number, in accordance with the track on which the sample is to be recorded. The samples are distributed in the tracks in a manner determined by these numbers. That is to say, samples which have the same segment number and the same channel number will be recorded on the same track.
FIG. 1 is a diagram illustrating the method of assigning channel numbers and segment numbers with that prior art shuffling technique, for the case of an NTSC video signal. As shown in FIG. 1, prior to recording, a field consists of 255 horizontal scanning lines (referred to in the following simply as "lines") each formed of 768 samples. First, the channel 0 and channel 1 samples of the field are separated, to be processed mutually separately. Then for each set of channel 0 and channel 1 samples, shuffling of samples is executed mutually separately within each of the lines, then error detection check bytes referred to as outer code check bytes are added to each line, with four of these outer code check bytes being added for every set of 1/6 of the samples of the line. For each channel, the resultant set of (sample+check byte) bytes is then divided into four segments, each consisting of 85 lines as indicated in FIG. 1. Shuffling is then executed of the digital video signal within each segment, independently of the other segments. Each of the resultant segments of each channel is then recorded on a separate track of a magnetic recording tape, with the resultant track pattern being as illustrated in FIG. 2, with the segments of channel 1 and channel 0 being recorded in successive alternation. In FIG. 2, S denotes the segment number, Ch denotes the channel number.
However with such a method, if there are occasional defects on the recording medium, and if a defect results in a burst error occurring whereby a plurality of successive samples recorded on the same track are lost, then a basic problem arises, which will be described referring to FIG. 3. Here, five successive samples of which occur in the same line of a field of the digital video signal are designated as 901 to 905. In the following description and in the appended claims, digital video signal samples which have the same relationship as that existing between the samples 901 and 903, or between the samples 903 and 905 in FIG. 3 will be referred to as "adjacent samples of the same line and same channel". With the prior art method of recording described above, adjacent samples of the same line and same channel will each have the same segment number and the same channel number. Hence, after shuffling has been completed, such adjacent samples of the same line and same channel will be recorded on the same track, e.g. as shown in FIG. 2.
If a burst error should occur along the longitudinal direction of a track, as shown by the hatched-line portion in FIG. 2, a plurality of successively recorded samples are lost. Considering any one of these samples, e.g. the sample 903, the adjacent samples of the same line and same channel as sample 903 (i.e. 901 and 905) may also be lost, with such a prior art recording method, as a result of the fact that all of these samples have the same segment number (0). Hence, it will not be possible to use these samples in interpolation processing to compensate for the loss of sample 903, i.e. complete implementation of the "retouching" processing described hereinabove becomes impossible. The same is true for any other sample which is lost as a result of a burst error occurring along one of the recording tracks, and this is a basic disadvantage of such a prior art recording method, which is caused by the fact that adjacent samples of the same line and same channel are recorded on the same track of the recording medium.
In a practical apparatus, when it is thus made impossible to execute retouching by using samples which have a very high correlation with a sample that has been lost, retouching processing may be executed by using samples which have a substantially lower degree of correlation with the lost sample. However when that is done, satisfactory compensation for samples that are lost as a result of read errors cannot be ensured.